I'd guess just over $90M or, 16k per seat.
That's based on an average 4.7% inflation rate over 3 years.
Now as I understand it, that upgrade included other stadium improvements. More bathrooms right?

Assuming you are only adding in seats to fill in corners, you would still have to do something with the circulation walks that protrude into the space where the new seating would go. The cost associated with replacing those circulation towers would certainly impact the overall costs.